The ornamental design for a wallhook, as shown and described.
FIG. 1 is a perspective view thereof;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a top plan view thereof;
FIG. 5 is a bottom plan view thereof;
FIG. 6 is a left side elevational view thereof; and,
FIG. 7 is a right side elevational view thereof.
The broken lines showing a strap, handle, zippers, zipper pulls and zipper tracks are for the purposes of illustrating environment and form no part of the claimed design.